GWEN IFILL: Now, from a conflict that may soon end to one that rages still in Syria.

William Brangham has this update on the five-year civil war there, and the international efforts to stop it.

WILLIAM BRANGHAM: Recriminations erupted in full force just one day after peace talks stalled in Geneva. Saudi Arabia and Turkey blamed heavy new attacks by Syrian government forces and Russian airstrikes for undermining the diplomatic effort.

The Syrian army confirmed today it expects it will soon surround the city of Aleppo, Syria's largest city. It's an advance that could have far-reaching consequences.

Faysal Itani is a fellow at the Atlantic Council in Washington.

FAYSAL ITANI, Fellow, Atlantic Council: You're seeing a general trend of regime victories. And Aleppo would be a sort of crowning achievement of this trend, thanks in large part to Russian air support. There has not yet been a complete encirclement of the city, but the main supply line that transfers ammunition, weapons, other supplies from Turkey, for example, to the rebels who are in Aleppo city has now been severed.

WILLIAM BRANGHAM: A British-based monitoring group reported today that, over the past week, Syrian and Russian warplanes have mounted one of the most intense bombing campaigns in months. They hit more than 500 opposition targets throughout Syria, paving the way for the offensive north of Aleppo.

That brought warnings today of a new mass exodus of refugees.

Turkey's prime minister spoke in London at a conference on aid for Syrian refugees.

AHMET DAVUTOGLU, Prime Minister, Turkey: Sixty thousand to 70,000 people in the camps in North Aleppo are moving towards Turkey. My mind is not now in London, but in our border, how to relocate these new people coming from Syria.

WILLIAM BRANGHAM: Turkey is already sheltering more than 2.5 million Syrians, making it the biggest refugee host country in the world. The Turkish prime minister insisted today that the U.S. take a stronger stance against Russian actions in Syria.

In turn, U.S. Secretary of State John Kasich said the Russian foreign minister has now agreed to at least consider a cease-fire.

JOHN KERRY, Secretary of State: We discussed and he agreed that we need to discuss how to implement cease-fire, and also how to get access by both parties. The opposition needs to allow access for humanitarian assistance, and the regime in Syria needs to allow access.

WILLIAM BRANGHAM: The depth of that need was made clear in this drone video of the devastation in the city of Homs after five years of civil war. It was shot by the film company RussiaWorks.

Secretary Kerry said the 70 donor nations gathered in London cannot turn away from the horror.

JOHN KERRY: If people are reduced to eating grass and leaves and killing stray animals in order to survive on a day-to-day basis, that is something that should tear at the conscience of all civilized people, and we all have a responsibility to respond to it.

WILLIAM BRANGHAM: The conference ultimately pledged some $10 billion over the next four years to assist Syria and countries like Jordan, Lebanon, and Turkey. That brought a glimmer of hope to some refugees housed at the sprawling Zaatari refugee camp in Jordan.

AHMED NAFEL, Syrian Refugee (through interpreter): God willing, this conference will help us. We need help to feed our children.

WILLIAM BRANGHAM: Still, there is little sign that the conflict will end soon. Russia today accused Turkey of planning its own invasion into Syria.

IGOR KONASHENKOV, Spokesman, Russian Ministry of Defense (through interpreter): We have serious grounds to suspect that Turkey is preparing for a military incursion. We have presented undeniable evidence to the international community proving Turkey opened artillery fire over a residential area to the north of Latakia.

WILLIAM BRANGHAM: And Saudi Arabia announced it's poised to send ground troops into Syria to join the fight against the Islamic State.